[Patch] Change named locales in the new codecvt tests

[Paolo Carlini]

Paolo Carlini wrote:

> I know that the whole approach wrt non-installed locales is
> in (positive!) flux, but I would like to change the chosen
> names to something actually installed by the last released
> version of the library.

Another short follow up: I have just learned that a few widespread
distributions (e.g., RH8) ship with the en_US.UTF-8 locale
installed and some other too besides those produced by the
standard `make localedata/install-locales'. This explains why
some people are only seeing failures for the /3.cc tests...

A this point, besides robustifying the testsuite wrt non-installed
localedata, which is a very good thing IMO, we should either
use in the testcases locales installed by default by stock glibc (as I
suggested yesterday), or explain in the docs where the necessary
localedata can be found/generated via localedef.

Paolo.